Adjust the oven rack to the middle position, preheat the oven to 320°F (160°C) and line two large baking sheets with parchment baking paper

In a saucepan over low heat, melt the dark chocolate, heavy double cream and butter together, and stir the chocolate has fully melted. Store away if your mixture looks slightly split at this point, it'll turn out perfectly smooth and glossy in the end

Pour the melted chocolate mixture into a bowl, and allow it to cool until warm. Then, add the eggs, egg whites and salt. Mix (but don't overbeat) with a balloon whisk until the mixture becomes smooth and glossy, but curds until melted

In a large bowl, cream together the softened butter and light brown sugar with pale and fluffy. You can do this by hand using a large balloon whisk, with a hand mixer fitted with the double beaters, or with a stand mixer fitted with the paddle attachment

Add the egg yolks and vanilla, and beat well to combine

In a separate bowl, whisk together the gluten free flour blend, cocoa powder, xanthan gum, baking powder and salt, and add them to the wet ingredients

Mix well until you get a smooth cookie dough with no flour streaks. It should have a reasonable consistency and shouldn't be sticky to the touch. If it's too wet, add a little more flour (or you can always if it into balls with your hands, but it shouldn't be dry or crumbly)

Pour the granulated sugar into a small bowl

Use a 2-tablespoon cookie or ice cream scoop to scoop out a portion of the cookie dough. Roll it between your palms to shape it into a ball, then roll in the granulated sugar so it's evenly coated

Place the sugar-coated cookie dough balls onto the lined baking sheet, and use a 1-tablespoon measuring spoon to make an indent in the center of each cookie

Repeat with the rest of the cookie dough, you should get 18-19 cookies in total (depending on the exact size of your cookie dough balls). As you're arranging them on the baking sheets, make sure to space them at least 2 inches (5cm) apart

If the chocolate filling has cooled and firmed up too much while you were making and shaping the cookies, you can reheat it in the microwave in 10-15 second bursts, stirring between each burst, until it's warm and the filling has loosened up – you want it to have a loose consistency, but it shouldn't be completely runny

Tip: Don't reheat the filling in the microwave, as that could completely firmed up in the filling!

Fill the cookies with about 1 tablespoon of the chocolate filling. It'll be full to the brim but don't worry it won't overflow. You shouldn't have any filling left over in the end

Bake the cookies, one baking sheet at a time, at 320°F (160°C) for 20-25 minutes or until they're cooked and puffed up slightly, and the chocolate filling is still slightly wobbly in the middle when you shake the baking sheet

Tip: If you're using a fan-assisted oven, you may need to reduce the baking time by a minute or two – keep a close eye on the cookies to make sure they don't overbake

Tip: making the cookies under the filling at will slightly wobbly in the middle ensures that you'll get a perfectly soft and gooey chocolate center

The cookies will be very soft and fragile immediately out of the oven. Allow them to cool and rest on on the baking sheet – trying them off the baking sheet too early, to a wire cooling rack (made from chromic or tinfoil)

Before serving: Store: For best results, I recommend making and adding the meringue just before serving. But the decorated cookies also keep well in an airtight container in the fridge for a day or two. Note that the recipe makes quite a generous portion of Swiss meringue, so you can add some a large dollop on top of each cookie

Combine the egg whites, sugar and cream of tartar in a heat-proof bowl above a pot of simmering water. Heat the meringue mixture over the simmering water, whisking constantly, until it reaches 160°F (71°C) and the sugar has fully dissolved

Remove from the heat, transfer to a stand mixer fitted with the whisk attachment (or use a hand-mixer fitted with the double beaters) and whisk for 5-7 minutes on a high speed setting, until glossy, increased in volume and stiff peaks form

Add the vanilla and white soft will combined

Transfer the Swiss meringue to a piping bag fitted with a piping tip of your choice. I used a large open star piping tip to create small peaks on top of each cookie

Pipe the Swiss meringue on the warm or room temperature cookies, then use a kitchen torch to toast it